This is a phase 1 key? The red key, not the black flip key?
With the black flip key, a broken button usually means that the microswitch underneath the rubber has come adrift from the circuit board, so it can be as easy to fix as to open up the key and solder (carefully - you have to know how to solder onto small printed circuits!) the switch back on. There's a full guide somewhere on the net. I have done it - it works.
With the phase 1 red key I don't know, though. Could be similar?
If you shake the key, do you hear something small rattling around inside?
If so, it's probably the microswitch. Even if you don't hear anything, it could still have cracked loose on one end.
Only one of the buttons has stopped working, right? If both buttons don't work, it's probably the battery that's died or the contacts from the battery to the circuit board that have shifted.
If it's only the one button, the procedure is basically as follows (as I said, there is a full guide with pictures somewhere on the web):
1. Open the battery compartment and remove the battery and its tray. Have a look in there to familiarise yourself with how the contacts are arranged.
2. Prise off the badge with a small, flat implement - it comes off fairly easily without gouging the black plastic - revealing a small screw.
3. Unscrew (careful not to strip it!) and the key can come apart in two halves. Be careful with the spring-loaded key blade. Look at how the spring works, as you'll need to put it back the same way.
4. Remove the insides of the key - once again looking at how the metal contacts for the battery are arranged.
5. Find the microswitch and solder it back into place, being careful not to bridge any other contacts with solder going astray.
6. Reassemble the key - you don't technically need to attach the key blade if you just want to test the button.
7. Test and see whether it works.
Mine didn't work on the first few tries, even though the boot opening button did work. I opened it back up again and carefully brushed the circuit board to clear any possible tiny solder spatters, reassembled again and then it did work. That was nearly two years ago and it's still working fine now.
What you could of course also try is to de-solder the button for the boot and put it in the place of the main button if you think the main button is broken.
You can buy those specific microswitches (I had the precise part number, but I forgot it, unfortunately - found it through internet research) in South Africa. It's a generic part available from electronics places. I think there was a a place in Midrand that had stock when I was doing inquiries before I got the key fixed.